It is known to use a low frequency tone superimposed on the normal working current at the power feed of a submarine cable system to enable a cable ship to locate the cable for inspection or repair using a submerged narrow band detector. It is normal to use a sine wave of typically between 4 and 25 Hz having a peak to peak amplitude of around 400 mA. The frequency response of the submarine cable is such that the toning signal is attenuated with distance and therefore not normally detectable beyond 1000 kilometres. The effect of attenuation is illustrated in FIGS. 7A to 7D for a sine wave toning signal which propagates through a series of four optical repeaters at a distance of 40 km (FIG. 7B), 80 km (FIG. 7C) and 160 km (FIG. 7D) from the powerfeed, respectively. As shown, after the fourth repeater the peak to peak amplitude of the toning signal is reduced to only around 3 mA.
The maximum amplitude of a toning signal generated by the power feed equipment is limited by the electrical characteristics of amplifiers within optical repeaters provided at intervals along the length of the submarine cable, which are sensitive to the negative half cycle of the toning signal. In future, repeaters will incorporate state of the art amplifiers which operate at much lower currents so that the expected maximum toning amplitude will be around 200 mA peak to peak. A toning signal at this amplitude would not be expected to propagate any great distance and still be detectable. Since ship time is expensive any decrease in toning distance could be costly.